The goal of this proposal is to assess the feasibility of an all-molecular method for activity-dependent feedback control of neuronal activity. We propose to generate calcium sensitive light emitting molecules (bioluminescent enzymes, luciferases) that drive light sensing optogenetic elements (channels or ion pumps, opsins) to control membrane voltage at the level of single cells for positive and negative feedback control. By adjusting calcium sensitivity and molecule location, light production can be made specific to large events such as bursts, or sensitive to individual spikes or single channel activity. By coupling these new luciferases to opsins, highly specific sensing of calcium at its source will trigger opsin activation for augmenting or suppressing neuronal activity, allowing a high degree of temporal and spatial precision in feedback control. Goals will be achieved by pursuing three aims: 1) Developing a calcium sensing split luciferase with significantly improved speed, brightness and range of sensitivity; 2) Targeting these new molecules to subcellular domains to enable highly specific biological outcomes; 3) Linking these new innovations to optogenetic readouts. Our strategy is non- invasive and it could be applied to large-scale manipulation of neural activity in behaving animals or in humans, where non-invasive, rapid feedback control of neuronal activity could be used to regulate clinically relevant activity in the brain. Our experiments are early stage, require proof of principle feasibility studies, but they have the potential to lead to a novel strategy to regulate activity only during periods of abnormal neuronal firing, such as attenuating runaway activity or amplifying local fluctuations. The molecular tools generated towards these feasibility experiments will be highly valuable in their own right, and achieving the goal of neural activity regulated self-control of neurons will be transformative.